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Quick introduction• WAL and replication internals.• Replication setup.
Troubleshooting tools• 3rd party tools.• Builtin tools.
Troubleshooting cases• Symptoms and problems.• Detection and solutions.• Lessons learned.
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03
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Quick introduction
01
Goals01
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Better understanding of streaming replication.
How to quickly find and fix problems.
https://goo.gl/Mm3ugt
Agenda01
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Write-Ahead Log.
Streaming replication internals.
Streaming replication setup.
Troubleshooting tools overview (3-rd party).
Troubleshooting tools overview (builtin).
Troubleshooting in practice.
Questions.
Write Ahead Log01
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Durability in ACID.
Almost all changes fixed in WAL.
pg_xlog/ (pg_wal/) directory in the DATADIR.
Synchronous WAL write by backends.
Asynchronous WAL write by WAL writer.
Recovery process relies on WAL.
Streaming replication internals01
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WAL Sender process.
WAL Receiver process.
Startup process (recovery).
Streaming replication vs. WAL archiving.
Streaming replication internals01
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WALBuffers Storage
WALSender
Network
StartupProcessStorage
WALReceiver
Streaming replication setup01
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Master:
● postgresql.conf;
● Restart.
Standby:
● pg_basebackup;
● postgresql.conf;
● recovery.conf setup.
Master setup01
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wal_level, max_wal_senders, max_replication_slots.
archive_mode, archive_command.
wal_keep_segments.
wal_sender_timeout.
synchronous_standby_names.
Master setup pitfalls01
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wal_level, archive_mode, max_wal_senders, max_replication_slots – require restart.
wal_keep_segments – requires extra storage space.
wal_sender_timeout – reduce that, if network is bad.
synchronous_standby_names – master freezes if standby fails.
Standby setup01
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hot_standby.
max_standby_streaming_delay, max_standby_archiving_delay.
hot_standby_feedback.
wal_receiver_timeout.
Standby setup pitfalls01
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hot_standby – enables SELECT queries.
max_standby_streaming_delay – increases max possible lag.
hot_standby_feedback:
● postpones vacuum;
● potential tables/indexes bloat.
wal_receiver_timeout – reduce that, if network is bad.
Recovery.conf01
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primary_conninfo and/or restore_command.
standby_mode.
trigger_file.
Recovery target:
● immediate;
● particular point/xid/timestamp;
● recovery_min_apply_delay.
Recovery.conf pitfalls01
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Any changes require restart.
Troubleshooting tools
02
3rd party troubleshooting tools02
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Top (procps).
Iostat (sysstat), iotop.
Nicstat.
pgCenter.
Perf.
3rd party troubleshooting tools02
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Top (procps) – CPU usage, load average, mem/swap usage.
Iostat (sysstat), iotop – storage utilization, process IO.
Nicstat – network interfaces utilization.
pgCenter – replication stats.
Perf – deep investigations.
Builtin troubleshooting tools02
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Statistics views.
Auxiliary functions.
pg_xlogdump utility.
Builtin troubleshooting tools02
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Statistics view:
● pg_stat_replication;
● pg_stat_databases, pg_stat_databases_conflicts;
● pg_stat_activity;
● pg_stat_archiver.
Builtin troubleshooting tools02
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Auxiliary functions:
● pg_current_xlog_location, pg_last_xlog_receive_location;
● pg_xlog_location_dif;
● pg_xlog_replay_pause, pg_xlog_replay_resume;
● pg_is_xlog_replay_paused.
Builtin troubleshooting tools02
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pg_xlogdump:
● Decodes and displays XLOG for debugging;
● Can give wrong results when the server is running.
pg_xlogdump -f -p /xlog_96 \
$(psql -qAtX -c "select pg_xlogfile_name(pg_current_xlog_location())")
Troubleshooting cases
03
Troubleshooting cases03
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Replication lag.
pg_xlog/ bloat.
Long transactions and recovery conflicts.
Recovery process: 100% CPU usage.
Replication lag03
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Main symptom – answers differ between master and standbys.
Detection:
● pg_stat_replication and pg_xlog_location_dif();
● pg_last_xact_replay_timestamp().
Replication lag03
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# \d pg_stat_replication View "pg_catalog.pg_stat_replication" Column | Type | Modifiers ------------------+--------------------------+----------- pid | integer | usesysid | oid | usename | name | application_name | text | client_addr | inet | client_hostname | text | client_port | integer | backend_start | timestamp with time zone | backend_xmin | xid | state | text | sent_location | pg_lsn | write_location | pg_lsn | flush_location | pg_lsn | replay_location | pg_lsn | sync_priority | integer | sync_state | text |
Replication lag03
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# SELECT client_addr AS client, usename AS user, application_name AS name, state, sync_state AS mode, (pg_xlog_location_diff(pg_current_xlog_location(),sent_location) / 1024)::int as pending, (pg_xlog_location_diff(sent_location,write_location) / 1024)::int as write, (pg_xlog_location_diff(write_location,flush_location) / 1024)::int as flush, (pg_xlog_location_diff(flush_location,replay_location) / 1024)::int as replay, (pg_xlog_location_diff(pg_current_xlog_location(),replay_location))::int / 1024 as total_lagFROM pg_stat_replication;
сlient | user | name | state | mode | pending | write | flush | replay | total_lag ----------+--------+-------------+-----------+-------+---------+-------+-------+--------+----------- 10.6.6.9 | repmgr | walreceiver | streaming | async | 0 | 0 | 0 | 410480 | 410480 10.6.6.7 | repmgr | walreceiver | streaming | async | 0 | 2845 | 95628 | 112552 | 211025 10.6.6.6 | repmgr | walreceiver | streaming | async | 0 | 0 | 3056 | 9496 | 12552 10.6.6.8 | repmgr | walreceiver | streaming | async | 847582 | 0 | 0 | 3056 | 850638
Replication lag03
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# SELECT client_addr AS client, usename AS user, application_name AS name, state, sync_state AS mode, (pg_xlog_location_diff(pg_current_xlog_location(),sent_location) / 1024)::int as pending, (pg_xlog_location_diff(sent_location,write_location) / 1024)::int as write, (pg_xlog_location_diff(write_location,flush_location) / 1024)::int as flush, (pg_xlog_location_diff(flush_location,replay_location) / 1024)::int as replay, (pg_xlog_location_diff(pg_current_xlog_location(),replay_location))::int / 1024 as total_lagFROM pg_stat_replication;
сlient | user | name | state | mode | pending | write | flush | replay | total_lag ----------+--------+-------------+-----------+-------+---------+-------+-------+--------+----------- 10.6.6.9 | repmgr | walreceiver | streaming | async | 0 | 0 | 0 | 410480 | 410480 10.6.6.7 | repmgr | walreceiver | streaming | async | 0 | 2845 | 95628 | 112552 | 211025 10.6.6.6 | repmgr | walreceiver | streaming | async | 0 | 0 | 3056 | 9496 | 12552 10.6.6.8 | repmgr | walreceiver | streaming | async | 847582 | 0 | 0 | 3056 | 850638
Replication lag03
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# SELECT client_addr AS client, usename AS user, application_name AS name, state, sync_state AS mode, (pg_xlog_location_diff(pg_current_xlog_location(),sent_location) / 1024)::int as pending, (pg_xlog_location_diff(sent_location,write_location) / 1024)::int as write, (pg_xlog_location_diff(write_location,flush_location) / 1024)::int as flush, (pg_xlog_location_diff(flush_location,replay_location) / 1024)::int as replay, (pg_xlog_location_diff(pg_current_xlog_location(),replay_location))::int / 1024 as total_lagFROM pg_stat_replication;
сlient | user | name | state | mode | pending | write | flush | replay | total_lag ----------+--------+-------------+-----------+-------+---------+-------+-------+--------+----------- 10.6.6.9 | repmgr | walreceiver | streaming | async | 0 | 0 | 0 | 410480 | 410480 10.6.6.7 | repmgr | walreceiver | streaming | async | 0 | 2845 | 95628 | 112552 | 211025 10.6.6.6 | repmgr | walreceiver | streaming | async | 0 | 0 | 3056 | 9496 | 12552 10.6.6.8 | repmgr | walreceiver | streaming | async | 847582 | 0 | 0 | 3056 | 850638
Replication lag03
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# SELECT client_addr AS client, usename AS user, application_name AS name, state, sync_state AS mode, (pg_xlog_location_diff(pg_current_xlog_location(),sent_location) / 1024)::int as pending, (pg_xlog_location_diff(sent_location,write_location) / 1024)::int as write, (pg_xlog_location_diff(write_location,flush_location) / 1024)::int as flush, (pg_xlog_location_diff(flush_location,replay_location) / 1024)::int as replay, (pg_xlog_location_diff(pg_current_xlog_location(),replay_location))::int / 1024 as total_lagFROM pg_stat_replication;
сlient | user | name | state | mode | pending | write | flush | replay | total_lag ----------+--------+-------------+-----------+-------+---------+-------+-------+--------+----------- 10.6.6.9 | repmgr | walreceiver | streaming | async | 0 | 0 | 0 | 410480 | 410480 10.6.6.7 | repmgr | walreceiver | streaming | async | 0 | 2845 | 95628 | 112552 | 211025 10.6.6.6 | repmgr | walreceiver | streaming | async | 0 | 0 | 3056 | 9496 | 12552 10.6.6.8 | repmgr | walreceiver | streaming | async | 847582 | 0 | 0 | 3056 | 850638
Replication lag03
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Network problems – nicstat.
Storage problems – iostat, iotop.
Recovery stucks – top, pg_stat_activity.
WAL pressure:
● pg_stat_activity, pg_stat_progress_vacuum;
● pg_xlog_location_dif().
Replication lag03
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Network/storage problems:
● check workload;
● upgrade hardware.
Recovery stucks – wait or cancel queries on standby.
WAL pressure:
● Reduce amount of work;
● Reduce amount of WAL:
● full_page_writes = of, wal_compression = on, wal_log_hints = of;
● expand interval between checkpoints.
pg_xlog/ bloat03
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Main symptoms:
● unexpected increase in the usage of the disk space;
● abnormal size of pg_xlog/ directory.
pg_xlog/ bloat03
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Detection:
● du -csh;
● pg_replication_slots, pg_stat_archiver;
● errors in postgres logs.
pg_xlog/ bloat03
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Problems:
● Massive CRUD.
● Unused slot.
● Broken archive_command.
pg_xlog/ bloat03
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Solutions:
● check replication lag;
● reduce checkpoints_segments/max_wal_size, wal_keep_segments;
● change reserved space ratio (ext filesystems);
● add an extra space (LVM, ZFS, etc);
● drop unused slot or fix slot consumer;
● fix WAL archiving;
● checkpoint, checkpoint, chekpoint.
Recovery conflicts03
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Main symptoms – errors in postgresql or application logs.
postgres.c:errdetail_recovery_conflict():● User was holding shared bufer pin for too long.
● User was holding a relation lock for too long.
● User was or might have been using tablespace that must be dropped.
● User query might have needed to see row versions that must be removed.
● User transaction caused bufer deadlock with recovery.
● User was connected to a database that must be dropped.
Recovery conflicts03
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Detection:
● pg_stat_databases + pg_stat_databases_conflicts;
● postgresql logs.
Recovery conflicts03
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Problems:
● queries are cancelled too often;
● long transactions on a standby – check pg_stat_activity;
● huge apply lag – check pg_stat_replication.
Recovery conflicts03
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Solutions:
● increase streaming delay (potentially causes lag);
● enable hot_standby_feedback (potentially causes bloat);
● rewrite queries;
● setup dedicated standby for long queries.
Recovery 100% CPU usage03
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Main symptoms:
● huge apply lag;
● 100% CPU usage by recovery process.
Recovery 100% CPU usage03
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Detection:
● top – CPU usage;
● pg_stat_replication – amount of lag.
Recovery 100% CPU usage03
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Investigation:
● perf top/record/report (required debug symbols);
● pg_xlogdump.
Recovery 100% CPU usage03
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Solutions:
● depend on investigation' results;
● change problematic workload (if found).
Lessons learned03
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Streaming replication problems are always distributed.
There are many sources of problems:
● system resources, app/queries, workload.
Always use monitoring.
Learn how to use builtin tools.
Links
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PostgreSQL official documentation – The Statistics Collectorhttps://www.postgresql.org/docs/current/static/monitoring-stats.html
PostgreSQL Mailing Lists (general, performance, hackers)https://www.postgresql.org/list/
PostgreSQL-Consulting company bloghttp://blog.postgresql-consulting.com/
